What are the conditions necessary for the successful transplantation of neural tissue to the surface of the brain? We have already defined two requisites: availability of target sites for transplanted neurons and a critical age of the donor. We are examining a third condition: revascularization of the graft. Although dissociated cells can survive in the cerebrospinal fluid, this fluid probably cannot sustain growth of organized tissue. The superior cervical ganglion (SCG) graft, about 1 Mum thick, must be revascularized in order to survive. We have been following the re-entry of new vessels into the grafts by filling the vessels with carbon black from the heart and with methacrylate casts of the vascular bed for examination with the scanning electron microscope (SEM). Vessels of the central nervous system (CNS) contain the enzyme gamma-glutamyl transpeptidase, detectable histochemically. We have found that the enzyme is not present in the vessels of normal SCG in situ. We can, then, distinguish between the two types of vessels and are processing grafts to see whether the CNS vessels persist or are replaced by the graft's own, intrinsic vessels. In 14 out of 18 grafts, new, patent, vessels invade the SCG allografts only 18 to 24 hours after transplantation. These vessels can be filled from the heart and, therefore, must anastomose rapidly with endothelial sprouts from the pia and choroid plexus. By 48 hours, this revascularization, in terms of total length of vessels, measured with a digitizer, is about 55 Mu/Mum square of tissue. If entire ganglia (about 2mm long) are grafted, a comparably rapid revascularization takes place. As in the SCG fragments, the new vessels usually form a network at the ganglion's periphery where the ganglion cells are situated. When grafts are inserted into the cerebellar substance, revascularization by parenchymal vessels is also brisk. Thus, the SCG graft exerts a rapid and pronounced angiotropic effect on all brain vasculature. Preliminary measurements with the Sokoloff method indicate that the permeability of the graft's vessels to the neutral amino acid, alpha-aminoisobutyrate, is between that of cerebral vessels and those of the choroids plexus.